1. Field of the Invention
This invention relates to a variable gain control circuit, and more particularly to an improved variable gain control circuit possible to linearly control a signal gain and effective for use in a matrix 4 channel stereo decoder.
2. Description of the Prior Art
In the prior art, a bi-polar transistor or Field Effect Transistor is used for a variable impedance device. Normally, the device is provided in parallel relationship to a signal transmitting path.
In such a case, the collector-emitter path of the bi-polar transistor or the drain-source path of the Field Effect Transistor is connected between the signal transmitting path and a reference point and the base or gate electrode is connected to a control voltage source. The bi-polar transistor or Field Effect Transistor has usually a predetermined threshold voltage, so that a control voltage supplied to the base or gate electrode cannot control the variable impedance at zero level thereof. Therefor, a DC biasing source is provided between the base and emitter electrodes or the gate and source electrodes for eliminating the defect. However, when the DC biasing source provided is such that the collector-emitter path or the drain-source path is biased in the forward direction, the control voltage applied to the base or the gate electrode is produced at the output terminal of the signal transmitting path with a predetermined amplified level so as to cause the quality of the output signal to be deteriorated.